(1) Field of the Invention
The present invention relates to a method of determining a sound localization filter for approximation of a head related transfer function, and also relates to a sound localization control system incorporating the sound localization filter.
(2) Description of the Related Art
A technique of sound localization is known. In this method, a pair of microphones are provided at the positions of the two ears of a dummy head to record the original sound emitted from a sound source in a first space where the dummy head is arranged. The reproduced sound, obtained by reproducing the recorded sound, is supplied to a pair of headphone speakers provided at the positions of the two ears of a listener. By using this method, the listener can hear the reproduced sound as if the source of that sound was located, in a second space where the listener stays, at the same position as that of the actual sound source in the first space. This technique is called the sound localization.
Japanese Laid-Open Patent Application No.2-298200 discloses a technique of sound localization control which uses either an analog filter or a digital FIR (finite impulse response) filter. In the method of the above publication, the amplitude and the phase of binaural signals are controlled through signal processing so as to control the sound localization. The original sound emitted from the sound source is analyzed in the frequency domain, and the frequency-dependent amplitude difference and phase difference are applied through signal processing to the binaural signals of right and left channels which are supplied to the headphone speakers of the listener. By using the method of the above publication, the localized position of a simulated sound source within the second space relative to the position of the listener can be shifted to a desired position through the signal processing. In other words, the sound localization can be controlled by using the method of the above publication.
In order to realize the sound localization control, a sound localization filter must be adapted for approximation of a head related transfer function. FIG. 1A and FIG. 1B are diagrams for explaining a head related transfer function used for the sound localization control.
FIG. 1A shows a binaural system having a dummy head provided in a first space. In the system of FIG. 1A, a pair of microphones of the R (right) and L (left) channels are provided at the positions of the two ears of the dummy head to record the original sound emitted from a sound source in the first space where the dummy head is arranged. The reproduced sound, obtained by reproducing the recorded sound, is supplied to a pair of headphone speakers of the R and L channels provided at the positions of the two ears of a listener in a second space.
FIG. 1B shows a binaural system including a pair of sound localization (S/L) filters 101 and 102. The S/L filters 101 and 102 are provided between the microphones of the first space and the speakers of the second space for approximation of right-channel and left-channel head related transfer functions HRTF-R and HRTF-L. The system of FIG. 1B simulates the functions of the system of FIG. 1A by using the S/L filters 101 and 102.
In the system of FIG. 1B, the original monaural signals originated by the actual sound source in the first space are processed through the S/L filters 101 and 102 so as to shift the localized position of the simulated sound source within the second space relative to the position of the listener, to a desired position. In order to realize the sound localization control, measurements of the frequency characteristics of the head related transfer functions (the HRTF-R and HRTF-L) for each of a set of predetermined direction angles about the front position of the listener are needed. In the system of FIG. 1B, a plurality of sets of filter coefficients of the S/L filters 101 and 102 which represent the measured characteristics for all the predetermined direction angles are retained in a memory, and one of the sets of filter coefficients is selected according to the desired direction angle for the localized position, so as to apply the selected coefficients to the S/L filters 101 and 102.
xe2x80x9cA Study on Clustering Method of Sound Localization Transfer Functionxe2x80x9d of the Institute of Electronics, Information And Communication Engineers (IEICE), EA9301 (1993.4), by S. Shimada and others, teaches a method of determining the sound localization transfer function by measurement of the impulse response of a digital filter to white noise generated in a given environment. FIG. 2 shows measurements of frequency characteristics of a head related transfer function with respect to a set of predetermined direction angles about the front position of a listener. In FIG. 2, the curve of 0xc2x0 indicates the measured frequency characteristics for the front position of the listener, and the curves of 0xc2x0 through 120xc2x0 indicate the measured frequency characteristics for the set of predetermined direction angles 0xc2x0 through 120xc2x0.
A sound localization (S/L) filter is realized by storing a plurality of sets of filter coefficients of a digital filter, which represent the measured filter characteristics, such as those of FIG. 2, for all the predetermined direction angles in a memory of a sound localization control system. One of the sets of filter coefficients stored in the memory is selected according to the desired direction angle for the localized position, so as to apply the selected coefficients to the digital filter. Hence, the sound localization control is possible by using the sound localization control system having the digital filter.
However, in a conventional sound localization control system having a digital filter, the sets of filter coefficients stored in the memory of the system are fixed to the measurements of the frequency characteristics of the digital filter in the given environment. It is impossible for the conventional sound localization control system to freely change the stored filter coefficients so as to suit the filter characteristics to various environments or the individual listeners.
Japanese Laid-Open Patent Application No. 5-252598 discloses a sound localization control system using a digital FIR (finite impulse response) filter. In the system of the above publication, a set of vectors of filter coefficients of the digital filter which represent typical filter characteristics, including the impulse responses of spatial transfer functions and the transfer functions of headphones, are obtained by using a clustering method of vector quantization, and such vectors of filter coefficients are stored in a database. However, the filter coefficients depend on the environments and the listeners used for the measurement, and it is difficult to change the stored filter coefficients so as to suit the filter characteristics to various environments or the individual listeners.
Further, the sound localization control system of the above-mentioned publication requires a large size of the hardware including the FIR filter and the database, and requires a computational complexity of signal processing. On the other hand, a digital IIR (infinite impulse response) filter can have a small size of the hardware with the coefficient memory, and makes it possible to easily change the stored filter coefficients so as to suit the filter characteristics to various environments or the individual listeners. However, a technique which designs a digital IIR filter for approximation of a transfer function with complex frequency characteristics, such as those of FIG. 2, is not yet established. In addition, it is desirable that the digital IIR filter is efficient in achieving the sound localization control. Generally, it is difficult to achieve complex frequency characteristics of a head related transfer function with a digital IIR filter, and a digital IIR filter is likely to become unstable due to limit cycle oscillation.
It has been reported that, when designing a digital IIR filter for approximation of a transfer function with complex frequency characteristics, such as those shown in FIG. 2, any simple frequency characteristics can be approximated by using a biquad digital filter (or a variable attenuation equalizer). One approach to designing a digital IIR filter for approximation of the head related transfer function is to perform the frequency transformation in the analog domain and then to convert the analog filter into a corresponding digital filter by a mapping of the s-plane into the z-plane. On the other hand, as disclosed in xe2x80x9cIIR Filter Designxe2x80x9d of the Interface, pp. 206-213, (1996.11) by H. Ochi, another approach is to directly designing an IIR filter in the frequency domain, which uses the sampling of frequency characteristics. However, this method requires the design of a high-order IIR filter and the order of the designed filter is not always constant.
An object of the present invention is to provide a novel and useful method of determining a sound localization filter for approximation of a head related transfer function in which the above-described problems are eliminated.
Another object of the present invention is to provide a sound localization filter determining method which determines a digital IIR filter for approximation of a head related transfer function, the digital IIR filter achieving smooth shifting of a localized position of a simulated sound source to another and achieving a small size of the hardware.
Still another object of the present invention is to provide a sound localization control system, incorporating sound localization filters for approximation of head related transfer functions of right and left channels, which achieves smooth shifting of a localized position of a simulated sound source to another by execution of a cross-fade function, and requires only a single IIR filter for one of the right and left channels.
The above-mentioned objects of the present invention are achieved by a sound localization filter determining method which includes the steps of: storing a plurality of sets of initial parameters with respect to a plurality of predetermined direction angles about a front position of a listener into a memory; reading one of the sets of initial parameters from the memory in accordance with a localization shift signal; calculating an optimum filter parameter based on the read initial parameters, the optimum filter parameter needed to approximate desired frequency characteristics of the head related transfer function; determining filter coefficients of the sound localization filter based on the optimum filter parameter; and supplying the determined filter coefficients to a coefficient buffer provided for the sound localization filter.
The above-mentioned objects of the present invention are achieved by a sound localization control system which shifts a localized position of a simulated sound source relative to a front position of a listener into a desired position in response to a localization shift signal and has a cross-fade function, the system including: a sound localization filter which inputs a sound signal and generates a localized sound signal based on filter coefficients and on the input sound signal, the filter having an input selector and an output selector; an input buffer which temporarily stores the input sound signal; a coefficient buffer which stores the filter coefficients of the filter; a first output buffer which temporarily stores the localized sound signal output by the filter when the filter is connected to the first output buffer via the output selector; a second output buffer which temporarily stores the localized sound signal output by the filter when the filter is connected to the second output buffer via the output selector; a fader, connected to the first and second output buffers, which provides the cross-fade function of the localized sound signals output from the first and second output buffers; and a control unit which replaces the filter coefficients stored in the coefficient buffer, with new filter coefficients by transmitting the new filter coefficients to the coefficient buffer when a localization shift signal is received, the control unit controlling the input and output selectors of the filter so as to connect the input buffer and the filter and connect the filter and one of the first and second output buffers, wherein the filter generates a new localized sound signal based on the sound signal stored in the input buffer and on the new filter coefficients stored in the coefficient buffer, and supplies the new localized sound signal to said one of the first and second output buffers via the output selector, the first and second output buffers outputting the localized sound signal and the new localized sound signal to the fader.
According to the sound localization filter determining method of the present invention, it is possible to achieve smooth shifting of the localized position of the simulated sound source to another with only a single IIR filter provided for one of the right and left channels. A sound localization control system incorporating the sound localization filter determined by the method of the present invention requires only a small size of the hardware. Further, the sound localization filter determined by the method of the present invention is effective in changing the stored filter coefficients in an arbitrary manner so as to adapt the filter characteristics to various environments or the individual listeners.
According to the sound localization control system of the present invention, it is possible to achieve smooth shifting of the localized position of the simulated sound source to another by execution of the cross-fade function with the right-channel and left-channel sound localization filters and the output buffers, and the sound localization control system of the present invention requires only a single IIR filter for one of the right and left channels. Further, the sound localization control system of the present invention is effective in achieving the execution of the cross-fade function with a small size of the hardware.